1. Technical Field
The present invention relates to an anti-frictional bearing constituting a rotationally supporting portion of a spindle motor of a hard disk drive device, VTR, and so on. The present invention further relates to a motor into which such a bearing is incorporated.
2. Description of the Prior Art
The compact hard disk device to be incorporated into a personal computer includes magnetic disk or disks driven by means of spindle motor in high rotational speed. The rotational member of such motor is adapted to be journalled through an anti-friction bearing having an inner diameter of 4-6 mm and an outer diameter of 8-15 mm.
Recently, a remarkable development or improvement is achieved in the hard disk device regarding the miniaturization and high packaging density. Especially for the hard disk device the size of which is equal to or less than 3.5 inch, the packaging density is increased rapidly. More recently, the hard disk device of the size of 2.5 inch to be incorporated into the hand held personal computer of the note book type is also demanded to have substantially the same memory capacity as that of the hard disk device of 3.5 inch in spite of its small size.
In order to enlarge the memory capacity of the hard disk device of the size of 2.5 inch, it is necessary to increase both of the track recording density and the track density. The presently demanded track density from 10 KTPI to 14 KTPI (TPI: Track Per Inch) can be satisfied by the track pitch less than 2.54 xcexcm. This value of the track pitch corresponds with the track density of 10 KTPI.
In either hard disk device tge size if which is 3.5 inch or 2.5 inch, it is necessary to increase the number of revolutions of the magnetic disk or disk to increase the data transfer rate of the hard disk device. For example, the hard disk device of 3.5 inch requires the number of revolution from 5400 rpm to 7200 rpm, and the hard disk device of 2.5 inch requires the number of revlution from 4000 rpm to 5000 rpm.
When it is intended to read or write datum accurately into the magnetic disk of increased track density, it is necessary to reduce the rotational run out of the magnetic disk. The rotational run out is apt to increase in proportion to the number of revolutions of the magnetic disk. It is therefore important in the high packaging density of the hard disk device to improve the precision of the rotational run out of the magnetic disk. It is therefore important in the high packaging density of the hard disk device to improve the precision of the rotational run out of the magnetic disk.
In order to reduce the rotational run out of the magnetic disk, it is necessary to reduce the run out attributable to the anti-frictional bearing itself of the spindle motor for driving the magnetic disk. The counter measures having been taken for the problem of the rotational run out of the magnetic disk are to improve the sphericity of the rotational bodies of the anti-frictional bearing and to effect the high precision working on the raceway surface of the inner and/or outer races to reduce the working tolerance to the minimum.
However, a microscopic undulation formed inevitably during working or processing on the raceway surface of the inner and/or outer race will change the relative position of the inner and outer races during the operation of the bearing. This changing of the relative position will cause the rotational run out. This rotational run out can be observed as an irregular run out in which the positional relationship between the inner race and the outer race is asynchronous with the rotation of the bearing. This run out is known as an asynchronous rotational run out referred to as NRRO (non-repeatable run out).
When the asynchronous rotational run out is increased beyond the allowable maximum extent, the magnetic head for reading and/or writing date can not be moved accurately relative to the magnetic disk of high tracking density. This will cause an error in effecting the reading and/or writing datum into disk. In conclusion, the asynchronous rotational run out will fail the reliability of the hard disk device.
In other words, the asynchronous rotational run out of the anti-frictional bearing to be incorporated into the spindle motor will interfere the high packaging density and the high speed of the hard disk device, i.e. the reduction of the asynchronous rotational run out of the anti-frictional bearing to be incorporated into the spindle motor is extraordinarily important in achieving the high packaging density and the high speed of the hard disk device.
The asynchronous rotational run out is influenced by the shape of the undulation on the raceway surface of the inner and/or outer races and the number of rotational bodies interposed therebetween. In order to reduce the asynchronous rotational run out, it is necessary to measure the out of roundness of the raceway surface accurately, to wake a harmonic analysis on thus obtained value of measurement as described below, and to select the inner and/or outer races which are suitable for the number of rotating bodies to be interposed therebetween. The harmonic analysis will now be described as follows.
Each of the inner and/or outer races has a raceway surface representing a complex undulation. This undulation can be seized as a function, the frequency of which is one revolution, i.e. the function is a composite of a plurality of harmonic vibrations.
In concretely, the shape of raceway surface as shown in FIG. 19(a) can be seized as a composite of a harmonic vibration of the frequency of ⅓ revolution (tertiary vibration) as shown in FIG. 19(b), a harmonic vibration of the frequency of {fraction (1/7)} revolution (seventh vibration) as shown in FIG. 19(c), and a harmonic vibration of the frequency of {fraction (1/50)} revolution (fifty vibration) as shown in FIG. 19(d).
In this connection, the undulation presented on the raceway surface can be expressed as a following function f(t) including a plurality of frequencies xcfx89, 2xcfx89, 3xcfx89 . . .
f(t)=C0+C1 cos (xcfx89t+xcfx861)+C2 cos (2xcfx89t+xcfx862)+C3 cos (3xcfx89t+xcfx863)+ . . . 
In the harmonic analysis, the constant C0, C1, C2, . . . , xcfx861, xcfx862, . . . are determined.
In the harmonic analysis effected on the shape of the raceway surface of the inner and/or outer races, the harmonic vibration of 1/n revolution forms a shape of an undulation including crests the number of which is n. In this connection, the harmonic vibration of 1/n revolution is referred to as the undulation including crests the number of which is n. The medial magnitude (C1, C2, . . . ) of the displacement amplitude of the shape varying in a sinusoidal manner is referred to as unilateral amplitude of each number of crests.
The shape of the raceway surface of each inner raceway, the out of roundness of which are 0.13 xcexcm, 0.096 xcexcm, 0.084 xcexcm, and 0.055 xcexcm is shown in FIGS. 20-23 respectively in the magnification of 100,000. The results of the harmonic analysis made on each shape of the raceway surface are shown in FIGS. 24-27 respectively.
The numbers put on the left column of each table are basic number N, the numbers to be added to the basic number N are put on the upper row of each table, and the values listed on the table are the values of unilateral amplitude.
For example, in the table of FIG. 24, the value put on the field of N+0 of the second row (N=7) leans that the component of vibration of {fraction (1/7)} revolution of the shape of the raceway surface as shown in FIG. 20 is 0.002 xcexcm, that is the unilateral amplitude in the case that the number of crests are seven is also 0.002 xcexcm. The value put on the field of N+1 of the second row means that the unilateral amplitude in the case that the number of crests are eight is 0.006 xcexcm, and the value put on the field of N+2 of the second row means that the unilateral amplitude in the case that the number of crests are eight is 0.005 xcexcm. The designation --- put on the fields of the table means that the unilateral amplitude can not be measured, i.e. there are substantially no unilateral amplitude.
The asynchronous rotational run out of the anti-frictional bearing relates to the shape of the undulation represented on the raceway surfaces of the inner and/or outer race and the number of rotating bodies as mentioned above. Particularly, it is known that the value of unilateral amplitude in the number of crests of aZ and aZxc2x11 (a is positive integer, and Z is the number of rotating bodies) will influence on the rotational run out.
This is caused by the run out due to the deflection between the rotating bodies and the positions of the crests. In this connection, when the anti-frictional bearing, including rotating bodies the number of which is Z, is intended to be manufactured, the inner or outer races greater in its value of unilateral amplitude can not be employed, since they will cause the rotational run out of the anti-frictional bearing.
Concretely, the number of rotating bodies to be utilized for the anti-frictional bearing of the spindle motor of the hard disk device of the size equal to or smaller than 3.5 inch is normally from 8 to 12. Explaining on the most general case that the number of rotating bodies is eight, the values of the unilateral amplitude put on each field of N+0 (the number of crest is seven), N+1 (the number of crest is eight), and N+2 (the number of crest is nine) of the second row of each of FIGS. 24-27 will influence on the rotational run out, so that it is necessary to reduce these values to 0.002 xcexcm or less than 0.002 xcexcm.
However, in each table of FIGS. 24-27, the values of the unilateral amplitude on the number of crests from seven to nine does not satisfy the above mentioned condition, so that the inner race representing the shape of the raceway surface as illustrated in FIGS. 20-23 are unsuitable for using in the anti-frictional bearing including eight rotating bodies.
As mentioned above, it is necessary in the prior art to make following cumbersome operations to obtain the anti-frictional bearing reduced in its asynchronous rotational run out and thus suitable for the spindle motor of the hard disk device. The above-mentioned operations are to make a measurement on the out of roundness of the inner and/or outer races, to make the harmonic analysis thereon, and to select the inner and/or outer races based on the relationship between the unilateral amplitude obtained by the harmonic analysis and the nailer of crests.
Accordingly the object of the present invention is to solve the problems of the prior art. In other words, the object of the present invention is to provide an anti-frictional bearing in which the asynchronous rotational run out is reduced substantially, the complex analyzing operation and the selecting operation to be carried on the inner and/or outer races are unnecessary, and high packaging density and high speed of the hard disk device can be assured when it is incorporated into the spindle motor of the hard disk device. Another object of the present invention is to provide a motor including such anti-frictional bearing.
These and other objects are achieved by an anti-frictional bearing including an inner raceway formed on an outer peripheral surface of an inner race, an outer raceway formed on an outer peripheral surface of an inner race, an outer raceway formed on an inner race peripheral surface of an outer race, and a plurality of rotating bodies interposed between the race ways and retained by retainers in a predetermined distance with each other, wherein an out of roundness of a raceway surface of at least one of said inner and outer race is equal to 0.05 xcexcm or less than 0.05 xcexcm.